1. Field of the Invention
This invention is in the field of propping agents or proppants used to hold open the fractures in subterranean formations around oil and gas wells created during hydraulic fracturing operations. More particularly it relates to a low density proppant, a method of making the low density proppant and a method of using the low density propping agent in shallow formation fracturing operations.
2. Description of the Related Art
Hydraulic fracturing is a technique intended to increase the productivity of an oil or gas well by creating conductive fractures or channels in the formation surrounding the well. First, a fluid is injected into the well at a sufficiently high rate to hydraulically fracture the underground formation thus creating and propagating a crack or fracture in the rock.
Next, a propping agent is added to the fluid and is pumped into the formation to “prop” open the newly formed cracks once the fracturing operation ceases. These cracks that are propped open facilitate the flow of oil or gas from the formation into the wellbore for recovery. Without the addition of the propping agent, the cracks would close under the lateral or overburden closure stress. The propping agent is pumped into the fractured formation in a slurry of fluid and propping agent. This propping agent must have sufficient strength to resist crushing by the closure stresses of the formation. The deeper the well depth, generally the stronger the proppant needs to be to resist crushing. Thus, the proppants used in shallower depths need not be quite as strong as the proppants used in deeper depths.
It has long been known that sintered bauxite having an alumina content of about 85% is strong enough to withstand crushing at well depths of greater than 20,000 feet. However, these high strength propping agents have high densities, i.e. apparent specific gravities above 3.4 g/cc, and require high viscosity pumping fluids or high pumping rates to keep them in suspension during the pumping operation. High strength proppants require larger pumping equipment and cause greater than normal wear on fluid carrying equipment. Because of the disadvantages associated with high density proppants, the proppant industry has created a variety of proppants with lower densities and less strength for use in shallower wells.
Intermediate density proppants, e.g., apparent specific gravity from about 3.1 to 3.4 g/cc, have been found to have sufficient strength to provide adequate permeability at intermediate depths and pressures. In these intermediate density proppants, the density was lowered primarily by lowering the alumina content to about 75%, as described in U.S. Pat. No. 4,427,068, which issued to Fitzgibbon. The intermediate density proppants are recommended for use in well depths of from about 8,000 to about 12,000 feet.
A low density proppant is described in U.S. Pat. No. 5,120,455, which issued to Lunghofer, using kaolin clay having a 50% alumina content. This low density proppant has an apparent specific gravity of less than 3.0 g/cc (actually 2.62 to 2.65 g/cc) and is used for well depths up to about 8,000 feet.
An even lower density proppant, having an apparent specific gravity of from 2.20 to 2.60 g/cc, is described in U.S. Pat. No. 5,188,175, which issued to Sweet, using a starting material having an alumina content of from 25% to 40%. As noted in U.S. Pat. No. 5,188,175, the reduced density means that the pumping fluid can be less viscous and the pumping rate can be lowered, both of which are cost saving features. Therefore, there is a desire in the industry for a proppant that has an even lower density that the Sweet proppant, i.e., apparent specific gravity of 2.10 g/cc or less.
As can be seen from the prior art, lowering the alumina content of the material generally results in a lower density proppant. However, there is a real problem with the strength of the proppant when the alumina content gets too low because the corresponding higher silica content causes significant loss of strength. Therefore, efforts to get an even lighter proppant by using lower alumina material have failed. Nevertheless, the industry still wants a very low density proppant, i.e., apparent specific gravity 2.10 g/cc or less, that is strong enough to be used in shallow wells, i.e., wells less than about 5000 feet.